1. Field of the Invention
The present invention relates generally to conversion of organic materials into biological plastics. Specifically, the present invention relates to a process for converting dry biomass wastes or inexpensive organic fuels, for example, into bioplastics suitable for use as biodegradable thermoplastics.
2. Description of the Prior Art
Municipal solid waste is similar to other forms of organic wastes (e.g., sewage sludges, manures, and agricultural and forestry residues) in that it is a very biochemically heterogeneous substrate that is only slowly metabolized by interactive, complex mixtures of microbes. High-yield, useful products are, at best, limited to methane. Alternatively, dry low-grade organic materials can be thermally gasified into a fairly homogeneous synthesis gas product, primarily comprised of CO and H.sub.2 with smaller amounts of CO.sub.2, CH.sub.4, H.sub.2 S, and other trace gases. Chemical energy conversion efficiencies can be as high as 80-85%. Inexpensive organic fuels, such as coal, petroleum, natural gas, peat, and shale, also can be readily thermally gasified to a similar synthesis gas product.
In the past, a few species or mixtures of bacteria have been described that are able to utilize CO and/or H.sub.2. However, their inability to utilize light energy necessitates that most of the chemical energy of the substrates is excreted in products such as acetate or CH.sub.4. Less than 6% of the substrate is converted into new cell material by the anaerobic metabolism of these microbes. In general, aerobic bacteria commonly convert about 25% of easily digestable substrates into new cell mass. A more complete conversion of synthesis gas substrates into cell mass product would clearly be beneficial.
Depending upon growth conditions, the newly-formed bacterial cell mass can be comprised primarily of protein, carbohydrate, or lipid constituents. Each constituent may have commercial value. Bacteria high in protein content are potentially capable of use as sources of animal feed and human food supplements. Carbohydrate materials derived from bacteria are potentially useful for their rheological properties and as emulsifiers. To date, lipid materials isolated from microbes have been commercially used only in niche markets.
A lipid material, poly-.beta.-hydroxybutyrate (PHB) is commonly synthesized by a number of different microbes and packaged into compact, 0.2-0.8 micron granules. Hydroxyvaleric acid, if synthesized or supplemented in the growth medium, can also be assimilated into the polymer, as can any other of a number of hydroxylated organic acids. With breakage of the microbial cells, the polymer granules are released. Granules have been determined to be better than 98% PHB or copolymer, the remainder being primarily adsorbed surface protein. The granules contain linear polyester chains with molecular weights up to 500,000 daltons or more. PHB is a high-modulus, natural plastic with a melting point of 170.degree. C. and physical properties similar to polystyrene. When 5% to 20% hydroxyvaleric acid is incorporated into the polymer (PHB-V), the melting point is lowered and the product is stronger and more flexible with properties similar to polypropylene. At 30% hydroxyvalerate, the bioplastic has physical properties similar to polyethylene. Being biological products, both PHB and PHB-V can be completely biodegraded by common microbes indigenous to soil and aquatic environments. These types of biologically-made polymers and copolymers are generically termed bioplastics.
Commercial production of PHB and PHB-V for specialty uses, such as for biodegradable sutures or for time-release drug delivery, is currently limited to a few companies, notably Imperial Chemical Industry, Ltd. (ICI) of England. Processing technology and applications for the polymers are already well established. Much of the technology has been adapted from the single-cell protein industry. Operating costs utilizing ICI's methodology are high due to the existing requirements for sterility and the large amounts of sugar, organic acid, ammonium ion, and other defined nutrients necessary for the development of the types of microbial cell mass that ICI employs. Efficiencies of sugar and organic acid conversion into polymer are 25-30%. A 10,000-ton plant using ICI's technology is estimated to produce PHB-V at $2 per pound, which compares to non-degradable petroplastics at about 60 cents per pound.
There has not heretofore been provided a technique or process for simple and effective conversion of inexpensive, heterogeneous organic materials into biological plastics.